Glass substrate for magnetic recording medium and its use

Abstract

An aspect of the present invention relates to a glass substrate for a magnetic recording medium, which is comprised of glass with a glass transition temperature of equal to or greater than 600° C., an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10 −7 /° C., a Young's modulus of equal to or greater than 81 GPa, a specific modulus of elasticity of equal to or greater than 30 MNm/kg, and a fracture toughness value of equal to or greater than 0.9 MPa·m 1/2 .

What is claimed is: 
     
       1. A glass substrate for a magnetic recording medium, which is comprised of glass comprising K 2 O within a range of 0 to 1 mole percent and having:
 a glass transition temperature of equal to or greater than 600° C., 
 an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10-7/° C., 
 a Young's modulus of equal to or greater than 81 GPa, 
 a specific modulus of elasticity of equal to or greater than 30 MNm/kg, and 
 a fracture toughness value of equal to or greater than 0.9 MPa m½, 
 wherein the glass further comprises Li 2 O and Na 2 O and has a molar ratio {K 2 O/(Li 2 O+Na 2 O+K 2 O)} of the K 2 O content to the total content of the alkali metal oxides is equal to or lower than 0.13. 
 
     
     
       2. The glass substrate for a magnetic recording medium according to  claim 1 , the sheet thickness of which is equal to or greater than 0.8 mm. 
     
     
       3. The glass substrate for a magnetic recording medium according to  claim 1 , which is a glass substrate for a magnetic recording medium employed in a magnetic recording apparatus in which a rotational speed is equal to or higher than 7200 rpm. 
     
     
       4. The glass substrate for a magnetic recording medium according to  claim 1 , wherein the glass has been subjected to chemical strengthening. 
     
     
       5. The glass substrate for a magnetic recording medium according to  claim 4 , wherein the chemical strengthening has been conducted with the use of melt of potassium nitrate and/or sodium nitrate. 
     
     
       6. The glass substrate for a magnetic recording medium according to  claim 1 , which is a glass substrate for a magnetic recording medium employed in a magnetic recording apparatus equipped with a dynamic flying height head. 
     
     
       7. The glass substrate for a magnetic recording medium according to  claim 1 , which is employed in a magnetic recording medium for energy-assisted magnetic recording. 
     
     
       8. The glass substrate for a magnetic recording medium according to  claim 1 , wherein an arithmetic average roughness, Ra, measured by an atomic force microscope at a resolution of 512×256 pixels over an area of 1 μm×1 μm on a main surface of the substrate is equal to or lower than 0.15 nm. 
     
     
       9. A magnetic recording medium, which comprises a magnetic recording layer on the glass substrate for a magnetic recording medium according to  claim 1 . 
     
     
       10. The magnetic recording medium according to  claim 9 , wherein the magnetic recording layer comprises a magnetic material the main component of which is alloy of Pt with Fe and/or Co, and the magnetic recording medium is a magnetic recording medium for energy-assisted magnetic recording. 
     
     
       11. A magnetic recording apparatus of energy-assisted magnetic recording system, which comprises:
 a heat-assisted magnetic recording head comprising a heat source heating at least a main surface of a magnetic recording medium, a recording element, and a reproduction element; and 
 a magnetic recording medium according to  claim 9 . 
 
     
     
       12. A glass substrate blank for a magnetic recording medium, which is comprised of glass comprising K 2 O within a range of 0 to 1 mole percent and having:
 a glass transition temperature of equal to or greater than 600° C., 
 an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10−7/° C., 
 a Young's modulus of equal to or greater than 81 GPa, 
 a specific modulus of elasticity of equal to or greater than 30 MNm/kg, and 
 a fracture toughness value of equal to or greater than 0.9 MPa m½. 
 
     
     
       13. The glass substrate for a magnetic recording medium according to  claim 1 , wherein the glass substrate contains SiO 2 , Al 2 O 3  and one or more of alkali metal oxides R 2 O, where R denotes Li, Na, or K. 
     
     
       14. A glass substrate for a magnetic recording medium, which is comprised of glass having:
 a glass transition temperature of equal to or greater than 600° C., 
 an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10−7/° C., 
 a Young's modulus of equal to or greater than 81 GPa, 
 a specific modulus of elasticity of equal to or greater than 30 MNm/kg, and 
 a fracture toughness value of equal to or greater than 0.9 MPa m½, 
 wherein (1) the glass substrate contains SiO 2 , Al 2 O 3  and one or more of alkali metal oxide R 2 O where R denotes Li, Na, or K and (2) the molar ratio {K 2 O/(Li 2 O+Na 2 O+K 2 O)} of the K 2 O content to the total content of the alkali metal oxides is equal to or lower than 0.13. 
 
     
     
       15. The glass substrate for a magnetic recording medium according to  claim 14 , the sheet thickness of which is equal to or greater than 0.8 mm. 
     
     
       16. The glass substrate for a magnetic recording medium according to  claim 14 , which is a glass substrate for a magnetic recording medium employed in a magnetic recording apparatus in which a rotational speed is equal to or higher than 7200 rpm. 
     
     
       17. The glass substrate for a magnetic recording medium according to  claim 14 , wherein a content of K 2 O in the glass is less than 3 mol percent. 
     
     
       18. The glass substrate for a magnetic recording medium according to  claim 14 , wherein the glass has been subjected to chemical strengthening. 
     
     
       19. The glass substrate for a magnetic recording medium according to  claim 18 , wherein the chemical strengthening has been conducted with the use of melt of potassium nitrate and/or sodium nitrate. 
     
     
       20. The glass substrate for a magnetic recording medium according to  claim 14 , which is a glass substrate for a magnetic recording medium employed in a magnetic recording apparatus equipped with a dynamic flying height head. 
     
     
       21. The glass substrate for a magnetic recording medium according to  claim 14 , which is employed in a magnetic recording medium for energy-assisted magnetic recording. 
     
     
       22. The glass substrate for a magnetic recording medium according to  claim 14 , wherein an arithmetic average roughness, Ra, measured by an atomic force microscope at a resolution of 512×256 pixels over an area of 1 μm×1 μm on a main surface of the substrate is equal to or lower than 0.15 nm. 
     
     
       23. A magnetic recording medium, which comprises a magnetic recording layer on the glass substrate for a magnetic recording medium according to  claim 14 . 
     
     
       24. The magnetic recording medium according to  claim 23 , wherein the magnetic recording layer comprises a magnetic material the main component of which is alloy of Pt with Fe and/or Co, and the magnetic recording medium is a magnetic recording medium for energy-assisted magnetic recording. 
     
     
       25. A glass substrate blank for a magnetic recording medium, comprising glass having:
 a glass transition temperature of equal to or greater than 600° C., 
 an average coefficient of linear expansion at 100 to 300° C. of equal to or greater than 70×10−7/° C., 
 a Young's modulus of equal to or greater than 81 GPa, 
 a specific modulus of elasticity of equal to or greater than 30 MNm/kg, and 
 a fracture toughness value of equal to or greater than 0.9 MPa m½, 
 wherein (1) the glass substrate contains SiO 2 , Al 2 O 3  and one or more of alkali metal oxide R 2 O where R denotes Li, Na, or K and (2) the molar ratio {K 2 O/(Li 2 O+Na 2 O+K 2 O)} of the K 2 O content to the total content of the alkali metal oxides is equal to or lower than 0.13.